GATE 2017 EC (EC2) – Question 49
An abrupt pn junction at x=0 is uniformly doped on both p and n sides. Its depletion-region width is W, and E(x) is the electric field in the x direction. Which figure represents the electric-field profile near the junction?
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Correct answer: (A) Option A in the figure
Explanation
Within each uniformly doped depletion region, charge density is constant. Poisson's equation $dE/dx=\rho/\epsilon$ therefore makes E vary linearly with x on each side.
The field is continuous at the junction and vanishes at both depletion edges, giving a triangular profile. Here the n side is on the left and the p side on the right, so the field points in the positive x direction, from ionized donors toward acceptors.
The positive triangular profile is **A**, rather than the negative triangle or either rectangular profile.